The primary objective of the current proposal is to develop new aerosol sampling inlets. These inlets, using an entirely new concept, will comply with size-selective aerosol sampling criteria better than the inlets currently being used. It is recognized that a better evaluation of the hazard is possible if particles which do not contribute to the hazard are excluded from the airborne mass concentration analysis. The criteria for inhalable, thoracic and respirable fractions of airborne particles, proposed by Soderholm (1989) for this purpose, have recently been adopted by ACGIH, CEN and ISO. However, it is difficult to sample aerosols in the workplace according to the revised criteria because of unsatisfactory performance characteristics of existing sampling inlets. The proposed inlets for both personal and stationary sampling will be developed using well established aerosol inertial impaction theory and unconventional structure. A prototype array of single stage impactors with different jet velocities will be developed in the Phase I study. The new inlets will not only provide much better overall cut-characteristics to comply with the revised criteria, but also will be simple, inexpensive, and easy to operate. Furthermore, the final modification and optimization of the inlet design using experimental results will lead to compact products which can be operated in a very dusty environment over a full work shift without degradation of performance due to overloading.